Application of Desmodesmus sp. algae in municipal wastewater treatment

In the last three decades, there has been a significant increase in interest in microalgae as a source of various valuable substances for the food, cosmetic, agricultural, and biofuel industries. Microalgae are also successfully used in the treatment of municipal and food industry wastewater to remove biogens and in industrial wastewater to remove heavy metals. Algal biomass subjected to fermentation can subsequently be used for biogas and bioethanol production or as fertiliser. Phototrophic algae utilise carbon dioxide from the air during photosynthesis, converting it into organic compounds. Under optimal conditions, their growth can be highly efficient. Compared to land-based crops, significantly smaller areas are required for their cultivation. The most promising algae for industrial use are green algae and cyanobacteria. In this study, Desmodesmus sp. algae, strain 5G-D3, obtained from the Culture Collection of Baltic Algae, were used for the removal of soluble forms of nitrogen and phosphorus from municipal wastewater. Four bioreactors with a capacity of 2 litres were sequentially filled with raw municipal wastewater: undiluted, and diluted to 40 %, 20 %, and 10 %. The dilution was intended to simulate wastewater that had undergone partial treatment in a conventional wastewater treatment plant. The algal suspension was then inoculated. The culture solutions were intensively aerated, and four Osram Biolux fluorescent lamps were used for lighting. The light intensity in the bioreactors was 4800–5500 lux, with a photoperiod of 14 hours per day. The experiment was continued 16 days until no further algal growth was observed. The optical density of the culture was measured daily at a wavelength of 663 nm, corresponding to the absorption peak of chlorophyll A. The concentrations of nitrate(V) ions, ammonium ions, and phosphate ions were determined using the spectrophotometric method. These parameters were measured before and after the experiment. The reduction in ammonium ion concentration was as follows: 72 % for raw wastewater, 79 % for the 40 % diluted sample, 55 % for the 20 % dilution, and 41 % for the 10 % dilution. Regarding phosphate ions, the reduction exceeded 99 % for all dilutions of municipal wastewater and undiluted wastewater. However, in the case of nitrate(V) ions, a concentration increase to 4.25 mg/l was observed at the 10 % dilution level, associated with cell death from the 9th-10th day of cultivation. For the other dilutions of municipal wastewater and undiluted municipal wastewater, this parameter remained slightly above 0.1 mg/l.